Many industrial processes use or produce solids that contain water in various amounts and in various states of bonding. Bonding strengths of water range from waters of crystallization, to waters of hydration, to xe2x80x9cfreexe2x80x9d water on the surface or in the pores of the material. Free water that is found on the surface or in the pore structure of many solids is bonded to the material by weak electrostatic forces. In contrast, waters of crystallization and hydration involve high energy ionic and covalent bonds, and the removal of such waters requires input energies beyond the scope of this invention.
The input heat energy required to break the bonding forces and remove the free water from the surface and pores of a material is generally less than required to break the covalent and ionic bonding forces of the waters of crystallization and hydration. This invention is directed to the removal of free water from porous materials such as aggregates, clays, minerals, cements, grouts, ceramic powders, and plastics.
Surface free water is water found on the surface of any material. A person touching or observing material having surface free water normally describes it as being xe2x80x9cwetxe2x80x9d. When this surface water is removed, the material is normally described as being xe2x80x9cdryxe2x80x9d. However, even so-called xe2x80x9cdryxe2x80x9d porous materials may contain substantial amounts of free water in their pore structure. Thus a material may have xe2x80x9cpore waterxe2x80x9d, independent of having any xe2x80x9csurface waterxe2x80x9d.
Surface water is bonded in a single planexe2x80x94the surface, while small pore nacelles bind pore water in the multiple planes of the pore. Thus the pore water is slightly more strongly bonded than is the surface water, which requires less heat energy to evaporate than pore water. When a material is heated, the surface water evaporates first, followed by the pore water.
Fine aggregates (sand) are used in the manufacture of hot mix asphalt (HMA). To enable the asphalt binder to coat all surfaces of the aggregate particles, the particle must be free of surface water. Thus, it is important to determine when the aggregate is free of surface water. This is the point at which the aggregate reaches the saturated, surface dry (SSD) condition. It is important to know the bulk specific gravity of the fine aggregates in order to perform mix designs based on volumetrics. It is also important to know the amount of asphalt binder that can be absorbed by the aggregate and the percentage of voids in mineral aggregate (VMA). To calculate this, it is necessary to determine the moisture condition of the aggregate.
The current test methods to determine the SSD point of an aggregate sample are set forth in American Society of Testing Materials (ASTM) testing standard C128 and corresponding American Association of State Highway Transportation Officials (AASHTO) testing standard T84. The methods used in these tests are informally and collectively referred to as tamper and cone methods, in which an operator manually dries the sample with a hair dryer and then makes a cone pile of aggregate. This process is repeated until the cone slumps or collapses. The percent moisture of the sample at this point of collapse is recorded as the SSD point. Results of such tamper and cone tests vary widely from laboratory to laboratory, because the test is empirical, subjective and operator dependant.
Rounded natural sands produce useful SSD data under the current test protocols. However, the new Federal Highway Administration""s SuperPave HMA mix design system favors the use of more angular manufactured aggregate sands. The current cone and tamper protocols do not produce useful SSD data for these angular manufactured fine aggregates, because the particles have a rough surface texture and the cones do not slump readily or reliably when the SSD point is reached.
Under a grant from the Federal Highway Administration, the National Center for Asphalt Technology (NCAT) devised a method to more accurately determine the SSD condition of manufactured fine aggregates. NCAT developed a device that is described in their paper xe2x80x9cDevelopment of a New Test Method For Measuring Bulk Specific Gravity of Fine Aggregatesxe2x80x9d by Kandhal et. al. given January 2000 at the Transportation Research Board 79th Annual Meeting.
The NCAT device comprises a closed solid wall sample drum that has an air stream entrance opening at one end and a small air stream exit opening at its opposite end. After loading the drum with a precise sample amount of aggregate, the drum is rotated and warm air flows through the opening and axially through the sample, where it evaporates and absorbs the free water. The humidified air is forced out the exit, where its humidity is continually measured. The humidity changes when the SSD is reached and is recorded. A small circle of screen cloth is placed over the exit to prevent loss of sample aggregate in the exit air stream. Because of the small size of the exit opening, the screen cloth becomes easily clogged and requires frequent clearing. Recognizing the shortcomings of their test apparatus, NCAT issued invitations to instrument design companies to develop a commercially viable SSD instrument.
The equipment devised in response to this invitation generally involved known high tech principles of measuring sand moisture content and was expensive. Examples of equipment of this type can be found in U.S. Pat. Nos. 5,397,994xe2x80x94Phare (moisture probe using a resonant electric circuit connected to a diode detector), U.S. Pat. No. 5,220,168xe2x80x94Adamski et al (utilizing measurement of light, having different wavelengths that are affected by moisture content, projected onto the surface of an aggregate sample), and U.S. Pat. No. 5,212,453xe2x80x94Koehler et al (measuring the dielectric constant of aggregate by measuring the time lapse of a pulsed signal through the aggregate, which is a function of moisture content). Equipment of these types is expensive and far more complex than the original simple NCAT equipment and would present a maintenance nightmare in the normal working environment of such testing equipment, which is in the field.
Thus, there is a need for a method and simple, reliable equipment for determining the SSD point of a fine aggregate sample, and for determining the moisture content of that sample.
It is therefore an object of this invention to provide a method and simple, reliable equipment for determining the SSD point of a fine aggregate sample, and for determining the moisture content of that sample.
This inventor recognized that, while the NCAT equipment is problematic, the operating principle, that exit air humidity readings noticeably change when the fine aggregate samples reach the SSD condition point, is sound and this point can be accurately determined.
This invention provides a testing device that includes a rotatable mesh drum for holding an aggregate sample. The drum is located in an enclosure that has an inlet for heated air and an outlet for humidified air. A fan supplies air that is heated by an electric heater and flows radially through the entire length of the rotating drum where it is humidified by water evaporated from the sample. This humidified air exits through the outlet where the temperature and humidity are continuously recorded. The rotation of the drum tumbles the aggregate sample. This, the use of mesh, and the radial airflow through the drum insure exposure of the entire sample to the heated air and prevent moisture pockets, which could produce false readings. The mesh drum also solves the clogging problem of the prior NCAT device.
The testing device is mounted on a scale to continuously monitor the weight change of the sample as moisture is evaporated out of the aggregate. A transparent window is provided in the housing to enable observation of the tumbling characteristics of the sample.
Periodic temperature and humidity readings are taken of the exit air to determine the SSD point. The drying of the sample is allowed to continue beyond the SSD point until weight readings show no more loss of free water. The final sample weight is recorded as totally dry weight. The total free water percent can then be calculated.
In one aspect, this invention features apparatus for determining the SSD point of an aggregate, comprising a sample drum for holding an aggregate sample having free water, said drum having spaced ends connected by a peripheral body that is sufficiently porous to allow air flow into and out of the drum, but contain the aggregate. A housing includes a closed chamber for the drum, an air inlet for allowing air to enter the chamber and flow radially through the drum, and an air outlet for enabling air within the drum to exit the chamber. The apparatus includes means for rotating the drum, means for heating inlet air and forcing the heated air into the chamber to evaporate and absorb the free water from the aggregate, and means for measuring the relative humidity of air exiting the chamber to enable determination of the SSD point of the sample.
Preferably, the drum body is made of a fine mesh, and the drum is supported by rollers
In another aspect, this invention features a method of determining the SSD of an aggregate sample, comprising the steps of
providing a sample drum having a peripheral body that is sufficiently porous to enable air flow through the drum, but contain the aggregate,
placing the drum in a closed chamber,
rotating the drum to tumble the aggregate,
flowing heated air into and through the drum to humidify the air by evaporating and absorbing free water from the aggregate,
exiting the moisturized air from the chamber, and
measuring the relative humidity of the exit air to enable determination of the SSD point of the sample.